1. Field of the Invention
This invention relates to an ink composition for an ink jet printer, and more particularly to a hot-melt ink composition which is solid at room temperature and is suitable for ink jet printing conducted at temperatures higher than room temperature.
2. Description of the Related Art
An ink jet printing method can achieve high speed printing without making a noise. Liquid ink compositions containing water or the like as their vehicle have been used with the ink jet printing method. In the case of printing using an aqueous ink composition, the ink composition soaks into recording paper to produce an image therein. Therefore, when paper into which the ink composition readily soaks is employed as recording paper, the ink composition tends to spread in the paper after printing. As a result, printed ink dots are blurred and their sharpness is lost, resulting in deterioration of the quality of a printed image.
To obtain a high quality image on recording paper independently of the quality of the paper, hot-melt ink compositions comprising as a vehicle a wax or the like which is solid at room temperature have been proposed as disclosed in U.S. Pat. Nos. 4,390,369, 4,484,948 and 4,659,383, and Japanese Patent Laid-Open Publication No. 58-108271. These ink compositions are solid at room temperature. When conducting printing, they are fused by heat, and droplets of the fused ink compositions are ejected onto recording paper. The droplets are then cooled and solidified, thereby forming ink dots on the recording paper. Such an ink jet printing method is called a hot-melt ink jet printing method.
The above hot-melt ink compositions, however, are inferior to the conventional aqueous ink compositions in preservation and handling of printed documents. This is because dots of the hot-melt ink compositions printed on recording paper are poor in resistance to abrasion, that is, they are readily abraded or deformed when frictional force or pressure is applied thereto.
In order to solve the above problem, there has been proposed a hot-melt ink composition comprising solid organic solvents, mainly a monoalkylbenzenesulfonamide, and a thermosetting resin having a hydroxyl number of 50 or more and an acid number of 10 or less, as disclosed in U.S. Pat. No. 4,820,346 and Japanese Patent Laid-Open Publication No. 62-48774.
The above hot-melt ink composition is capable of producing an image with improved resistance to abrasion as compared with those hot-melt ink compositions which contain as a main component a natural wax. However, the abrasion resistance of printed dots of the above ink composition is still unsatisfactory when compared with that of printed dots formed by an electrophotographic method which is now superseding other conventional printing methods. Printed documents such as official documents are now required to have higher quality than ever, so that improvement in abrasion resistance of a printed image seems to be one important object.
When conducting printing, a hot-melt ink composition, which is solid at room temperature, is placed in a printer, and then fused. After completion of the printing, the ink composition is cooled, and solidified. Since the hot-melt ink composition is exposed to the above cycle, it tends to undergo thermal deterioration which is caused by a polymerization reaction, change in properties by heat, or the like. As a result, some troubles arise, for instance, the life of the ink composition is shortened, and a head nozzle of the printer gets clogged by the ink composition solidified due to change in properties.
In addition, the hot-melt ink composition comprising as its main component a monoalkylbenzenesulfonamide, in particular, ortho-toluenesulfonamide, gives rise to, with the passage of time, a phenomenon in which the surfaces of printed dots of the ink composition seem to be covered with white powder (hereinafter referred to as "blooming"). This phenomenon makes the printed image look poor.